Back in September and October 2009 we published the original and very popular Wideband Oxygen Sensor Controller. This was designed for use with the Bosch LSU4.2 wideband oxygen sensor. In this substantially revised design, we use the much-improved Bosch LSU4.9 sensor which supersedes the LSU4.2. This has necessitated an upgraded microcontroller, the addition of a sensor to monitor exhaust pipe pressure and a re-designed LED display module.

Most modern vehicles include a narrowband oxygen sensor so that the engine control unit (ECU) can control the air/fuel ratio. Unfortunately, that sensor is only accurate when the fuel/air mixture is stoichiometric, ie, when the mixture is exactly right to give complete combustion and with all the oxygen used in the burning process.

The engine control unit (ECU) normally adjusts the fuel mix to maintain an oxygen sensor signal that’s close to 450mV, the stoichiometric point. In practice, a narrowband sensor has a very sharp voltage change around the stoichiometric point and so the sensor voltage is continually cycling above and below 450mV as the ECU maintains the fuel mixture.

This is referred to as “closed loop” operation. It does not matter to the ECU that the narrowband sensor is inaccurate and non-linear outside closed loop operation.

To explain further, Fig.1 shows the typical output from a narrowband oxygen sensor. It has a very sharp response either side of the stoichiometric point (lambda of 1), ranging from about 300mV up to 600mV; the classic “S” curve. For rich mixtures, it ranges from around 600mV to almost 900mV (lambda up to 0.8), is quite non-linear and varies markedly with temperature. It is similarly non-linear for lean mixtures, ranging from around 300mV down to a few mV (lambda of about 1.15).

To learn about lambda, refer to the explanatory panel later in this article.

The ECU uses its own factory preset values to set rich mixtures for acceleration or lean for cruise conditions. This is referred to as “open loop” operation because the oxygen sensor is not capable of providing accurate feedback about the actual fuel mixture.

Now if you haven’t changed anything on your vehicle, then there is little reason to worry about the actual fuel mixtures at any time; the ECU takes care of it all. But if you have made any changes to the vehicle to improve its performance (eg, inlet air filter, throttle body and plenum, injectors, MAP or MAF sensor, custom ECU chip, supercharger or turbocharger, catalytic converter, exhaust manifold, mufflers and resonators, in short, anything that’s likely to result in significant changes to fuel mixtures and oxygen sensor readings) then you need a wideband oxygen sensor and a companion controller.